The characteristic of measuring large diameters is that the measuring apparatus is usually with large dimension and heavy weight, which leads to many problems such as much difficulty of operation in situ, complexity of calibration, large deformation and high cost. The roller way (Page 21-22, Volume 1, Foreign Metrology, 1991) provided a method to measure large diameters with compact measuring apparatus. It is necessary to include many parts in such method, such as the roller, badge, sensor of revolution, sensor of compaction, disk of glass grating, pulse counter, microprocessor, keyboard, digital-displaying device and so on. The diameter of the workpiece is obtained by counting the revolution of the known-diameter roller that is contacted with and rotated purely following the workpiece, when the workpiece rotates a whole or several whole numbers of revolution. The method suffers the following disadvantages. (1) Since the slippage of the roller on the surface of the workpiece is inevitable, it usually suffer the problem of bad repeatability and limited accuracy, which is the vital disadvantage of this method. (2) The measuring apparatus is complex and expensive. (3) Because the workpiece must revolve while measuring, it can only be employed on machine. (4) The workpiece to be measured must has a whole circumference.